The short answer to both questions is: it depends. Helpful, I know. Different schools do things differently. Often the job advert may let you know whether a lesson observation will form part of the interview process. Certainly when you're invited to interview you'll be given details of any lesson observations needed. If you've read the threads on these forums before, you'll realise that often the information given about the students is minimal. You will be able to ring for more info I should think but often they'll be looking to see what you can come up with and how you'll plan for what you find when you get there, ie have you got back up activities ready in case you find they are more or less able than you'd predicted.